Distribution of breakpoints induced by etoposide and X-rays along the CHO X chromosome

Abstract

SORB (selected observed residual breakpoints) induced by ionizing radiation or endonucleases are often non-randomly distributed in mammalian chromosomes. However, the role played by chromatin structure in the localization of chromosome SORB is not well understood. Anti-topoisomerase drugs such as etoposide are potent clastogens and unlike endonucleases or ionizing radiation, induce DNA double-strand breaks (DSB) by an indirect mechanism. Topoisomerase II (Topo II) is a main component of the nuclear matrix and the chromosome scaffold. Since etoposide leads to DSB by influencing the activity of Topo II, this compound may be a useful tool to study the influence of the chromatin organization on the distribution of induced SORB in mammalian chromosomes. In the present work, we compared the distribution of SORB induced during S-phase by etoposide or X-rays in the short euchromatic and long heterochromatic arms of the CHO9 X chromosome. The S-phase stage (early, mid or late) at which CHO9 cells were exposed to etoposide or X-rays was marked by incorporation of BrdU during treatments and later determined by immunolabeling of metaphase chromosomes with an anti-BrdU FITC-coupled antibody. The majority of treated cells were in late S-phase during treatment either with etoposide or X-rays. SORB induced by etoposide mapped preferentially to Xq but random localization was observed for SORB produced by X-rays. Possible explanations for the uneven distribution of etoposide-induced breakpoints along Xq are discussed.

γH2AX Foci Form Preferentially in Euchromatin after Ionising-Radiation

“…This was presumably due to reduced protection of DNA from radical damage in decondensed chromatin associated with a reduced local concentration of histones and other proteins and molecules that scavenge free radicals. A considerable body of published work exists that compares the frequencies of radiation induced CAs originating in heterochromatin versus euchromatin, (see for example [36] and references within), but there is no consensus as to whether radiation induced CAs occur with higher or lower than expected frequencies in heterochromatin. Notably though, a recent study has shown no difference in the frequency of γ-radiation-induced chromosome breaks between the largest block of heterochromatin in the human genome (1cen-1q12) and a similarly sized euchromatic region [21].…”

DNA Double Strand Breaks and Chromosomal Aberrations

“…Breakpoints in human chromosome 5 analyzed with mBAND FISH after exposure of cells to X-rays or to high-energy iron ions showed slight deviations from random distributions [Johannes et al, 1999;. Data on intrachromosomal breakpoint localizations indicate that DSBs occur preferentially in accessible chromatin areas irrespective of where these are located in chromatin domains [Martinez-Lopez et al, 2004Folle, 2008]. Recent molecular studies suggest indeed that the repair mechanisms may be different for heterochromatic and euchromatic regions.…”

Effects of Valproic Acid on Radiation-Induced Chromosomal Aberrations in Human Lymphocytes

“…[ 21 22 ] Transcriptionally active genomic regions, comprising an acetylated and relaxed chromatin structure, have proved to be more sensitive to ionizing radiation. [ 23 24 25 26 27 28 ] On the other hand, cells are also more radiosensitive when the chromatin achieves a high state of compaction during the G2/M phase of the cell cycle although G2/M cells do not get more DNA double-strand breaks (DSBs). [ 29 ] Therefore, even though a more relaxed chromatin structure favors the production of radiation-induced DNA strand breaks, DNA repair could be an important component of radiosensitivity as well.…”